[RFC 2/3] Documentation pps.rst: add PPS generators documentation

[Rodolfo Giometti <giometti@enneenne.com>]

This patch adds some examples about how to register a new PPS
generator in the system, and how to manage it.

Signed-off-by: Rodolfo Giometti <giometti@enneenne.com>
---
 Documentation/driver-api/pps.rst | 40 ++++++++++++++++++++++++++++++++
 1 file changed, 40 insertions(+)

diff --git a/Documentation/driver-api/pps.rst b/Documentation/driver-api/pps.rst
index 78dded03e5d8..c71b3b878e41 100644
--- a/Documentation/driver-api/pps.rst
+++ b/Documentation/driver-api/pps.rst
@@ -202,6 +202,46 @@ Sometimes one needs to be able not only to catch PPS signals but to produce
 them also. For example, running a distributed simulation, which requires
 computers' clock to be synchronized very tightly.
 
+To do so the class pps-gen has been added. PPS generators can be
+registered int the kernel by defining a struct pps_gen_source_info as
+follows::
+
+    static struct pps_gen_source_info pps_gen_dummy_info = {
+            .name                   = "dummy",
+            .use_system_clock       = true,
+            .get_time               = pps_gen_dummy_get_time,
+            .enable                 = pps_gen_dummy_enable,
+    };
+
+Where the use_system_clock states if the generator uses the system
+clock to generate its pulses, or from a peripheral device
+clock. Method get_time() is used to query the time stored into the
+generator clock, while the method enable() is used to enable or
+disable the PPS pulse generation.
+
+Then calling the function pps_gen_register_source() in your
+initialization routine as follows a new generator is created into the
+system::
+
+    pps_gen = pps_gen_register_source(&pps_gen_dummy_info);
+
+Generators SYSFS support
+------------------------
+
+If the SYSFS filesystem is enabled in the kernel it provides a new class::
+
+    $ ls /sys/class/pps-gen/
+    pps-gen0/  pps-gen1/  pps-gen2/
+
+Every directory is the ID of a PPS generator defined in the system and
+inside you find several files::
+
+    $ ls -F /sys/class/pps-gen/pps-gen0/
+    dev  enable  name  power/  subsystem@  system  time  uevent
+
+To enable the PPS signal generation you can use the command below::
+
+    $ echo 1 > /sys/class/pps-gen/pps-gen0/enable
 
 Parallel port generator
 ------------------------
-- 
2.34.1